1. Field of the Invention
This invention relates to an improved system for protecting structures, e.g., buildings, from the effects of seismic disturbances. More particularly, the present invention relates to improvements in damping systems, employable independently and/or in combination as a damping subsystem with a base isolated structure stabilization system.
2. Description of the Related Art
As described in the present inventor's U.S. Pat. No. 4,860,507 (hereinafter the '507 patent), a structure stabilization system can include a base isolation system in combination with either or both of a releasable interlock subsystem and a damping subsystem. The disclosure of this '507 patent is expressly incorporated herein both by reference and as effectively directly incorporated herein in its entirety.
Generally, the described base isolation system employs vertical support columns suspended by flexible elements from corresponding bases and which provide "floating" support of a structure relative to its foundation, thereby minimizing horizontal movement transmission from the ground to the structure. The base isolation system prevents unpredicted stresses from developing in the support columns and possible tendencies of the structure to rotate relatively to the foundation while assuring that a predetermined natural period of oscillation is maintained in common for all such columns and elements, and yet affording the ability to adjust the actual length of the flexible elements so as to maintain a common elevation of the support columns, thereby compensating for variations in ground level support of the bases, unequal stretching of the flexible suspension elements, and the like.
More particularly, the floors of the structure are individually connected to and supported by the plurality of vertical support columns. The support columns are suspended at their respective, lower ends by corresponding bases fixed to the foundation (and thus immovable with respect to the foundation and the earth in which the foundation is formed). Each base includes a plurality of generally vertical support members, each of which can sustain its respective, proportionate share of the compressive force of the structure, including potential lateral or transverse forces which may act thereon as produced by a seismic disturbance and which typically are much smaller than the compressive force. The vertical support members are secured at their lower ends to a base ring and at their upper ends to a support ring, to form an integral, rigid and strong structure. The support column passes downwardly through the support ring and preferably includes an enlarged diameter base portion. Flexible suspension support elements, e.g., cables, extend at their upper ends through corresponding through-holes in the support ring and are secured thereto by adjustable support mechanisms. The cables are secured at their lower ends to a metal clamp which is secured about the base of the support column. The cables further are engaged, each at a position intermediate its length, by an adjustable gripper mechanism which is secured to the support column. This adjustable gripper mechanism maintains the identical "effective" free length of the cables, thereby maintaining common harmonic characteristics, while permitting adjustment of the actual suspension lengths of the cables. This base isolation system minimizes the transmission of horizontal movement from the ground (and thus through the foundation of the structure) to the structure, in the event of a seismic disturbance. Effectively, the base isolation system permits the structure to "float" with respect to its foundation.
The releasable interlock subsystem employs a single pin received in a plate integrally formed in the floor of the structure, which prevents translational movement of the structure relative to its foundation in response to forces of ordinary levels, such as produced by winds. An automatic release mechanism is responsive to forces above a predetermined threshold level, as produced by a seismic disturbance, for automatically withdrawing the pin and causing the structure to "float", as supported by the base isolation system, thereby minimizing the transmission of horizontal movement from the ground to the structure.
Finally, the damping subsystem described in the '507 patent employs hydraulically interconnected dampers, arranged as one or more pairs, the dampers of each pair being mechanically connected in respectively inverted relationship between, and on corresponding, opposed sides of, the structure and its foundation (or other support fixedly secured to the ground). From a functional or theoretical standpoint, it is sufficient that the dampers of a given pair be displaced by substantially equal (and preferably the maximum possible) distances and in opposite directions from the center of gravity of the building. Preferably and typically, the dampers of each such pair are connected, in the relatively inverted relationship and at centrally located locations, between respective opposing, parallel walls of the structure and the corresponding foundation walls. A single such pair of dampers, mechanically connected in inverted relationship between the structure and its foundation and hydraulically interconnected, suffices to impede relative horizontal rotation, i.e., angular displacement, between the structure and its foundation, and furthermore will damp linear displacement of the structure relative to the foundation in a direction parallel to the orientation of the dampers. As a practical matter and preferably, a second such pair of dampers, oriented in a perpendicular or orthogonal direction relatively to the first pair and connected to corresponding opposing foundation walls orthogonally related to the first set of opposing foundation walls, is employed to damp linear displacement of the structure in the corresponding, perpendicular or orthogonal direction.
More particularly, reference is made to FIG. 1 herein which shows one embodiment of the damping subsystem of the '507 patent and, more particularly, one of two orthogonally related damper pairs. A first damper 10 of the pair is mechanically connected between a bracket 12 integral with a foundation wall 14 and a bracket 16 attached to the building floor 18, whereas the second damper 20 of the pair is oriented in inverted relationship, relatively to the first damper 10, and mechanically connected between a bracket 22 secured to the floor 18 and a bracket 24 integral with the opposing foundation wall 14. The dampers 10 and 20 are so connected at positions generally centrally disposed along the respectively associated structure/foundation walls.
The dampers 10 and 20 have corresponding pistons 11 and 13, the respective piston heads 11a and 13a of which define corresponding subchambers 10a, 10b and 20a, 20b within the dampers 10 and 20 and are connected to respective piston rods 11b and 13b. The pistons 11 and 13 are movable in sealed relationship within the corresponding cylindrical interior sidewalls of the respective dampers 10, 20 against the pressure of hydraulic fluid contained therein, in response to the forces tending to produce relative linear movement between the structure and the foundation walls 14 and which forces are transmitted to the dampers 10, 20 through the respective brackets 12, 16, 22 and 24. The dampers 10 and 20 are also hydraulically interconnected through hydraulic lines 30 and 32. The line 30 connects through orifice 38 with chamber 10a of damper 10 and through orifice 40 with chamber 20a of damper 20. In like manner, line 32 connects through orifice 42 with chamber 10b of damper 10 and through orifice 44 with chamber 20b of damper 20. Valves 34, 35 may be employed in the hydraulic line 30, and like valves 36, 37 may be employed in line 32 to regulate damping by selectively restricting fluid flow in the respective hydraulic lines 30, 32 between the dampers 10, 20.
Operation of the dampers 10, 20, under the influence of a seismic disturbance, will now be described. More particularly, for a structure having a base isolation system which does not afford the characteristics of the improved base isolation system of the '507 patent, the horizontal projection of the structure's center of gravity commonly does not coincide with the centroid of the forces which oppose the horizontal displacement of the earth. As a result, forces acting on the structure as produced by a seismic disturbance may produce rotation of the structure relative to its foundation and which rotation, in turn, produces linear displacements between the structure and the foundation, which occur in opposite directions along the opposing foundation/structure walls on opposite sides of the axis of rotation. These displacements are proportional to the distances of the respective walls from the center of rotation of the structure, and add to the linear displacement produced by translation of the structure relative to its foundation along one of these opposed foundation/structure walls. The increased linear displacement is a matter of serious concern, since it increases the probability of physical impact between elements attached to the structure and interconnecting and supporting same with respect to its base or foundation. The damper subsystem, since employing hydraulically interconnected damper pairs having respective, inverted connections between the foundation and the structure, prevents such gyrations.
It may be assumed that pistons 11 and 13 are normally centrally located within their respective dampers 10 and 20 and thus define corresponding, identical subchambers 10a, 10b and 20a, 20b and wherein the displacements h.sub.1 and h.sub.2 (later discussed) are equal to each other. Due to a seismic disturbance and as specifically illustrated in FIG. 1, bracket 22 (attached to the structure) and bracket 24 (attached to foundation wall 14) have moved more closely together than the opposite brackets 12, 16. The resulting movement of the piston 13 increases the pressure within and expels fluid from chamber 20a, decreasing the distance h.sub.1 from the end of the piston head 13a to the opposing wall of the damper 20. The hydraulic fluid, since essentially incompressible, travels through line 30 into the subchamber 10a of damper 10 and expands the volume of same, thereby driving piston 11 downwardly (i.e., in the orientation of FIG. 1) and correspondingly increasing the distance h.sub.2 from the piston head 11a to the opposing wall of the damper 10 and relatively displacing bracket 16 (attached to the floor 18) from bracket 12 (attached to the foundation wall 14). At all times, therefore, the sum of the distances h.sub.1 and h.sub.2 is constant.
Correspondingly, a seismic disturbance causing a downward displacement of bracket 16 relative to bracket 12, as viewed in FIG. 1, produces increased pressure within subchamber 10b which is communicated through line 32 to subchamber 20b, which then interacts between the piston 13 fixed to the bracket 24 (and in turn to the foundation wall 14) and the housing of damper 20, tending to draw the damper 20 and its associated bracket 22 (attached to the floor 18 of the structure) downwardly as viewed in FIG. 1. Again, the sum of h.sub.1 and h.sub.2 remains constant.
In summary, the above-described damping subsystem of the '507 patent is intended to achieve two important functions:
(1) to impede rotation or gyration of the structure relative to its foundation, in both the engaged and released states of the releasable interlock system--i.e., to maintain h.sub.1 +h.sub.2 =a constant; and
(2) to reduce the acceleration response of the system and damp lateral displacement of the structure relative to its foundation, when released from the releasable interlock system and thus when floating on the base isolation system.
The first function is achieved particularly by the hydraulic interconnection of the dampers of each pair, along with the relatively inverted mechanical connections of the respective dampers of the pair between the structure and its foundation. The second function is performed by the controlled damping characteristics of the dampers. In many cases, this conventional damping system of the '507 patent is fully applicable and effective.
However, depending on the building's characteristics (e.g., size and weight) and on the expected earthquake activity, the friction developed in the hydraulic lines interconnecting the dampers of a pair may exceed the maximum friction required to obtain optimal damping. Further, while generally it may be assumed that the hydraulic fluid employed in the damping system of the '507 patent is incompressible or at least that, in many cases, compressibility is negligible, one has to consider the dimensional factors involved. Particularly, due to the length of the interconnecting hydraulic lines 30, 32 for very large structures, the compressibility of the hydraulic fluid, expansion of the conduits in response to internal pressure and friction effects may reduce the responsiveness and controllability of the damping function. The '507 patent describes various additional features which attempt to compensate for the compressibility effect; for example, aligned and interconnected cylinders 46, formed of aluminum or other light metal and which have far less compressibility than the fluid, may be placed in and occupy a substantial portion of the volume within the lines 30, 32. While effective in performance, the cylinders 46 add to the cost and complexity of the overall system.
Thus, there remains a need for an improved damping system of simplified structure and reduced cost and complexity. For example, such an improved damping system should minimize the number of fluid paths and the number of fluid path interconnections, both to avoid frictional problems as above-noted and as well to avoid or minimize the problem of leakage at interconnections between components of the hydraulic system; furthermore, it should optimize the effectiveness of the control valves thereby to provide more precise regulation of the extent of damping.
The improved damping system of the present invention overcomes the deficiencies and inadequacies of prior art such damping systems and affords the above-described, advantageous and desirable characteristics, either when used as a subsystem of an overall structure stabilization system and/or as an independent damping system for structural stabilization.